When light metal, such as aluminum, is cut on a machine tool, e.g., a machining center, the motor for driving a spindle is required to provide high output power at high speed in order to improve cutting efficiency while allowing a highly accurate cut surface to be produced. In addition, the motor of the machining center or the like may be required to produce high torque at low speeds in order to permit this single machine to perform both cutting operations and one or more additional machining operations such as drilling and threading (tapping). Accordingly, when an induction motor is employed to drive the spindle, the induction motor is required to have a characteristic curve which provides both high torque at low speeds and high output power at high speeds, e.g., a 3 kg-m constant torque at not more than 3,500 RPM and 22 Kw constant output power at cutting speeds of about 20,000 RPM to 25,000 RPM. A typical motor characteristic curve for the induction motor with the characteristics described above is illustrated in FIG. 14.
One way to provide the necessary characteristics, i.e., high torque at low speeds and high output power at high speeds, using a single motor, is illustrated in Japanese Utility Model Publication No. 2559 of 1989. As shown in FIG. 15, for example, each phase of the stator windings of the induction motor is provided with two windings C1U' and C2U', C1V' and C2V' and C1W' and C2W', respectively. The various phases are selectively controlled by two three-phase contractors 1M and 2M, wherein the first contactor 1M includes contacts 1MU, 1MV and 1MW and second contactor 2M includes contacts 2MU, 2MV and 2MW. By energizing only the three-phase contactor 1M during low speed operations, the windings C1U' and C2V', windings C1V' and C2V' and windings C1W' and C2W' are Y-connected in series. By energizing only the three-phase contactor 2M during high speed operations, the windings C1U', C1V' and C1W' are delta-connected while the windings C2U', C2V' and C2W' are idle.
As should be clear from FIG. 15, however, the apparatus is low in winding operating efficiency because the C2U', C2V' and C2W' windings are not employed in the high-speed operation range. The motor control system of FIG. 15 is further complicated due to the fact that it requires the three-phase contractors 1M and 2M to switch between as many as nine lead wires for the stator windings of the motor. It should be noted that the motor of FIG. 15 requires a circuit for controlling the opening and closing of the three-phase contractors 1M and 2M, which further hinders efforts to reduce both the size and weight of the motor and associated control apparatus as well as the overall cost of the device.
It will be appreciated that the induction motor can also be controlled by slip-frequency vector control to provide the speed versus output characteristic and the speed versus torque characteristic shown in FIG. 16. In FIG. 16, the torque T.sub.0 is constant in the range 0.ltoreq.N.ltoreq.N.sub.b and the output power P.sub.0 is constant at N.sub.b .ltoreq.N. N.sub.b is referred to as the base speed. In particular, assuming that the primary current of the induction motor is divided into a magnetic flux current id and a torque current iq, controlling the air gap magnetic flux .PHI. (secondary flux linkage) and the torque current iq with respect to the speed, as shown in FIG. 17, provides the output characteristics shown in FIG. 16. In FIG. 17, the air gap magnetic flux .PHI. is assumed to be .PHI..sub.0, which is constant at or below speed N.sub.b, while the air gap magnetic flux .PHI. is assumed to be indicated by the following expression at speeds in excess of N.sub.b : ##EQU1##
It is also assumed that the torque current iq is iq.sub.0, which is constant in the overall velocity range.
FIG. 18 is a control block diagram of a known motor drive control apparatus under slip-frequency vector control. Referring to FIG. 18, the numeral 1 indicates a three-phase commercial AC power supply connected via a three-phase PWM inverter 2 to an induction motor 3 acting as a load. A speed detector 4 and current detectors 5a, 5b and 5c are connected to motor 3. Also shown in FIG. 18 are a speed controller 6, a torque current limiting circuit 9, a magnetic flux command circuit 10, a torque current controller 11, a slip angular frequency computing unit 12, an adder 13, an integrator 14, a magnetic flux computing unit 15, a magnetic flux controller 16, a magnetic flux current controller 17, a two phase-to-three phase converter 18 and a three phase-to-two phase converter 19. The operation of these units are discussed in greater detail below.
In FIG. 18, .omega..sub.r * indicates a speed command, .omega..sub.r represents a speed detection signal, iqs* and iq* designate torque current commands, iq denotes a torque current detection signal, .omega..sub.s indicates a slip angular frequency signal, .omega..sub.1 designates a primary current angular frequency signal, .theta..sub.1 denotes a phase signal, .PHI.* represents a magnetic flux command, .PHI. indicates a magnetic flux detection signal, id* denotes a magnetic flux current command, id designates a magnetic flux current detection signal, vq* represents a torque voltage command, v.sub.n * indicates a magnetic flux voltage command, V.sub.u *, V.sub.v * and V.sub.w * denote three-phase voltage commands, and i.sub.u, i.sub.v, and i.sub.w designate three-phase current detection signals.
The operation of the control apparatus shown in FIG. 18 will now be described.
The three-phase current detection signals, i.sub.u, i.sub.v, i.sub.w detected by the current detectors 5a, 5b, 5c, respectively, are input to the three phase-to-two phase converter 19, which then outputs the torque current detection signal iq and magnetic flux current detection signal id under the control of the input three-phase current detection signals i.sub.u, i.sub.v, i.sub.w.
The speed command .omega..sub.r * and the speed detection signal .omega..sub.r detected by the speed detector 4 are input to the speed controller 6, which amplifies the difference between the input speed command .omega..sub.r * and speed detection signal .omega..sub.r and outputs the torque current command iqs*. This torque current command iqs* is provided as the input to the torque current limiting circuit 9.
If the input torque current command iqs* is smaller than a broken-line value corresponding to the speed detection signal .omega..sub.r in FIG. 17, the torque current limiting circuit 9 outputs the value of the torque current command iqs, intact as the torque current command iq*. In FIGS. 16 and 17, the horizontal axis is represented as speed N but speed N and speed detection signal .omega..sub.r are substantially equivalent since .omega..sub.r =2.pi..times.N.
If the input torque current command iqs is larger than the broken-line value corresponding to the speed detection signal .omega..sub.r in FIG. 17, the torque current limiting circuit 9 outputs the broken-line value in FIG. 17 as the torque current command iq*. In other words, the torque current limiting circuit 9 operates to output the torque current command iq* limited to not more than the broken-line value in FIG. 17.
The torque current command iq* and torque current detection signal iq are input to the torque current controller 11, which then amplifies the difference between the input torque current command iq* and torque current detection signal iq and outputs the torque voltage command vq*.
The magnetic flux current detection signal id is input to the magnetic flux computing circuit 15, which calculates air gap magnetic flux .PHI. (secondary flux linkage) generated by the magnetic flux current, under the control of the input magnetic flux current detection signal id, according to the following expression: ##EQU2## where S is a differential operator, R.sub.2 is secondary resistance of the induction motor, L.sub.2 is secondary inductance, and M is primary/secondary mutual inductance.
The torque current detection signal iq and magnetic flux detection signal .PHI. are input to the slip angular frequency computing circuit 12, which then carries out operation according to the following expression (3) under the control of the input torque current detection signal iq and magnetic flux detection signal .PHI., and outputs the slip angular frequency signal .omega..sub.s. ##EQU3##
The slip angular frequency signal .omega..sub.s and speed detection signal .omega..sub.r are input to the adder 13, which then adds the input slip angular frequency signal .omega..sub.s and speed detection signal .omega..sub.r, and outputs the primary current angular frequency signal .omega..sub.1. This output primary current angular frequency signal .omega..sub.1 is then input to the integrator 14.
The integrator 14 then generates the phase signal .theta..sub.1 under the control of the input primary current angular frequency signal .omega..sub.1.
The speed detection signal .omega..sub.r is input to the magnetic flux command circuit 10, which then outputs the magnetic flux command .PHI.* which is equal to the continuous-line value in FIG. 17 corresponding to the input speed detection signal .omega..sub.r.
The magnetic flux command .PHI.* and magnetic flux detection signal .PHI. are input to the magnetic flux controller 16, which then amplifies the difference between the input magnetic flux command .PHI.* and magnetic flux detection signal .PHI. and outputs the magnetic flux current command id*.
The magnetic flux current command id* and magnetic flux current detection signal id are input to the magnetic flux current controller 17, which then amplifies the difference between the input magnetic flux current command id* and magnetic flux current detection signal id, and outputs the magnetic flux voltage command vd*.
The torque voltage command vq* magnetic flux voltage command vd* and phase signal .theta..sub.1 are input to the two phase-to-three phase converter 18, which then outputs the three-phase AC voltage commands V.sub.u *, V.sub.v * and V.sub.w * in accordance with the input torque voltage command vq* and magnetic flux voltage command vd*.
The three-phase AC voltage commands V.sub.u *, V.sub.v *, V.sub.w * are input to the PWM inverter 2, which then exercises PWM control of alternating current output by the three-phase commercial AC power supply 1 under the three-phase AC voltage commands V.sub.u *, V.sub.v * V.sub.w * and supplies PWM-controlled AC power to the induction motor 3 serving as a load.
As described above, the induction motor 3 can be vector-controlled under the predetermined speed command .omega..sub.r * and operated to provide the characteristics shown in FIG. 16.
The loss W.sub.LOSS generated in the induction motor is represented by the following expression: EQU W.sub.LOSS =W.sub.c1 +W.sub.c2 +W.sub.I (4)
That is, the loss W.sub.LOSS is represented by a sum of a primary copper loss W.sub.c1 attributable to the resistance of the stator windings, a secondary copper loss W.sub.c2 attributable to the resistance of the rotor windings (bars of the rotor in a squirrel-cage induction motor), and an iron loss W.sub.I collectively generated in the cores of the stator and rotor. It will be apparent that the loss attains its maximum value in the speed range centered on the base speed N.sub.b, e.g., in the speed range A shown in FIG. 16.
The components of the loss W.sub.LOSS generated in an induction motor designed to operate, as shown in FIG. 19(A), at a base speed of 7,000 RPM, constant torque of 3 kg-m at less than or equal to 7,000 RPM and constant output power of 22 Kw above 7,000 RPM but below 25,000 RPM are as indicated in FIG. 19(B) with respect to typical speeds. FIG. 19(B) indicates that the loss W.sub.LOSS of the induction motor reaches a maximum value of 2610 W when the base speed N.sub.b is 7,000 RPM.
The dimensions of the induction motor, i.e., motor diameter and length, and the sizing of an associated cooler device are determined so that the losses can be dissipated. In other words, the dimensions of the induction motor are determined by the base speed and the torque at that base speed. Thus, as the motor torque increases, the motor size increases.
According to another aspect of the motor control system illustrated in FIG. 18, the setting range of the air gap magnetic flux value .PHI..sub.0 in the low-speed range, which is shown in FIG. 17, must inevitably be determined based on permissible air gap magnetic flux density and motor shape i.e., motor diameter and length. Initiating torque current iq in the induction motor causes the motor-generated torque to increase in proportion to iq, while, at the same time, causing the motor-generated loss W.sub.LOSS to increase in the constant, low-speed range. Because of the limited capability for dissipating the losses generated, iq is limited, which, in turn, restricts the induction motor-generated torque and output power characteristics.
As described above, the prior art shown in FIG. 15 allows a single induction motor to achieve characteristics wherein large torque is required but comparatively low output power may be provided at low speed and high output power is required at high speed. However, the prior art results in decreased motor winding use efficiency and an increased number of motor lead wires, and further requires multiple contractors. Thus, the motor and the cooling apparatus are hindered from being reduced in size and increased in reliability.
Also, the prior art, which drives the induction motor under slip-frequency vector control to provide the speed-versus-output characteristic and speed-versus-torque characteristic shown in FIG. 16, causes motor size to be larger due to a relationship between loss W.sub.LOSS near base speed N.sub.b and a heat dissipation characteristic for dissipation of the loss W.sub.LOSS, whereby the motor or the apparatus is hindered from being reduced in size and increased in reliability.